Note: Descriptions are shown in the official language in which they were submitted.
CA 02723906 2010-11-09
WO 2010/015496 PCT/EP2009/059098
Mixing device having induction heating
The invention concerns a mixing device comprising a container for
accommodating material to be mixed, a mixing tool arranged in the interior
of the container and a heating device for heating the material to be mixed.
Mixing, that is to say combining at least two starting materials
having different properties to provide a mixture of substances, is a
fundamental operation in mechanical industrial process engineering.
The aim when using a mixing device is to achieve the highest
possible level of homogeneity of the new substance. To achieve the
desired homogeneity for example mixing devices having a rotating mixing
container are used.
In such devices at least one mixing tool is arranged generally
eccentrically in the interior of the container. When the container rotates
the material to be mixed which is accommodated in the container is
transported towards the mixing tool and thoroughly mixed by means
thereof. With such mixers, materials of any kind and consistency can be
processed quickly and with a high quality.
Experience has shown that a mixer having a rotating container mixes
without noticeable separation of the constituents of the mixture as
complete rotational turn-over of the material is achieved during a
revolution of the mixing container.
As many mixing processes are linked to a chemical reaction which
presupposes the supply of a given activation energy, it is already usual for
many situations of use for the material to be mixed to be heated during
mixing. That is also necessary if thermal liquid removal is to take place in
superposed relationship during the mixing operation.
Therefore to permit heating of the material to be mixed in the mixer
some mixing devices have a heating casing which surrounds the container
wall and is in contact therewith. Heat energy can then be fed into the
mixing container by way of the heating casing.
Depending on the
respective heat transfer medium the heating casing is either in the form of
a double casing for hot water or thermal oil heating or, when higher design
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pressures are involved for vapour heating, the heating casing is in the form
of a casing with projections thereon or with half-tubes welded thereto. The
known solutions are limited in respect of their maximum wall temperature
due to the maximum permissible temperature of the heat transfer medium
or in the case of vapour heating due to the necessary compression strength
of the double casing.
In addition it is not possible with those solutions to implement fast
heating curves up to very high wall temperatures as the heating rate is
limited by the heat transfer coefficient from the liquid or the vapour phase
to the container wall and the pressure losses and thus the flow speed of the
heat transfer medium in the heating casing.
Therefore having regard to the background of the described state of
the art the object of the present invention is to provide a mixing device of
the kind set forth in the opening part of this specification, which permits
the fastest possible heating of the material to be mixed, preferably even to
temperatures of greater than 200 C.
According to the invention that object is attained in that the
container at least partially comprises an electrically conductive preferably
metallic material and the heating device has at least one coil which can be
excited by an electric alternating field and which is so arranged that eddy
currents are produced in the electrically conductive material of the
container by the magnetic field change which occurs when the current flow
changes. Those eddy currents provide an increase in the temperature of
the container. That inductive heating of the container permits targeted,
direct heating of the container and thus the material to be mixed without a
heat transfer medium having to be previously heated. In addition the
temperature increase which can be achieved is limited only by the power of
the coil and not the chemical-physical properties of the heat transfer
medium.
In a preferred embodiment provided in the interior of the container
in the proximity of the container wall and/or the container bottom is at
least one stripping device which is movable relative to the container wall.
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In the simplest case the stripping device is static so that the necessary
relative movement is produced only by rotation of the container.
The stripping device provides for a continuous vertical component in
the flow of material to be mixed and prevents material adhering to or
baking on the container wall and/or bottom. In addition with a concentric
emptying opening in the bottom of the mixing container, the emptying
operation at the end of the mixing time is speeded up.
In a further particularly preferred embodiment the stripping device
has a temperature sensor for detecting the temperature of the material to
be mixed. The stripping device has proven to be particularly suitable for
accommodating a temperature sensor to detect the temperature of the
material to be mixed as the stripping device is in direct contact with the
material. Alternatively the step of determining the temperature of the
material to be mixed can also be effected by way of a temperature sensor
which is introduced separately from the stripping device into the mixing
chamber and which is contact with the product or which is contact-less.
A further preferred embodiment provides that there are provided at
least two separately regulatable heating devices, wherein preferably both
heating devices respectively have at least one coil which can be excited by
an electric alternating field and which are so arranged that eddy currents
are produced in the electrically conductive material of the container by the
magnetic field change which occurs when the current flow changes.
To ensure heating of the material to be mixed, that is as uniform as
possible, or to adapt the heating power in accordance with the thermal
flows into the material to be mixed, it may be advantageous for many
situations of use if there are provided a plurality of separately regulatable
heating devices, for example one or more of the heating devices can be
designed for heating the container bottom and one or more heating devices
can be designed for heating the container wall. In addition heating devices
can also be provided for heating the mixing tool and/or the stripping
device.
In that respect the power of the heating device which is designed to
heat the container wall can be the same as or also greater than or less than
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the power of the heating device for heating the container bottom. In that
case the distribution of power between the wall and the bottom preferably
approximately corresponds to the surface area ratio between the preferably
cylindrical wall surface to be heated directly and the surface of the
container bottom that is to be heated directly, preferably a surface in the
form of a circular ring or a circular surface. In
another particularly
preferred embodiment the power distribution to the effectively operative
heat transfer surface is based on the material to be mixed in the interior of
the container. In
a vertically arranged rotating mixing container the
bottom surface is usually almost completely covered with material. The
heat flow thus takes place over the entire heated bottom surface. In a
mixing container which is inclined with respect to the horizontal, having a
stationary stripping device, up to about 10-20 % of the bottom surface is
not directly acted upon with product, depending on the respective
arrangement of the stripping device. In the surfaces which are not directly
acted upon with product there is thus a markedly lower heat flow which
results from the heat loss due to radiation and convection at that location.
A comparable aspect applies to the wall surfaces, wherein in addition the
material filling directly contacts only a part of the heated wall surface in
the
stationary condition of not moving relative to the container wall while the
part of the heated wall surface, that is disposed thereabove, is in contact
with the material to be mixed which is thrown against the wall by the
mixing tool, and is thus used for heat transfer.
By way of example, with a surface area ratio between the wall and
the bottom of 3:1, a heating power distribution of at least 1:1, preferably
at least 1.5:1 and quite particularly preferably at least 2:1, is
advantageous.
The container is advantageously of a rotationally symmetrical
configuration, wherein a preferred embodiment provides that the heating
device for heating the container bottom is so arranged that an outer
portion, in the form of a circular ring, of the container bottom is not
substantially heated by the heating device, wherein the portion in the form
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=
of the circular ring is preferably of a width which is greater than 5 % and
particularly preferably greater than 10 % of the container diameter.
In other words the excitable coil is arranged parallel to the container
bottom but not in relation to the described portion in the form of the
5
circular ring. That ensures that no overheating of the container occurs in
the region of the container corner, if at the same time the container wall is
heated with a second heating device and the field lines of the two coils are
concentrated in the corner of the container.
Alternatively or in combination therewith, the heating device for
heating the container bottom can be so arranged that an inner circular
portion of the container bottom is not substantially heated by the heating
device, wherein the circular portion is preferably of a diameter greater than
30 %, particularly preferably greater than 50 %, of the container diameter.
In other words the excitable coil is arranged in a plane substantially
parallel to the container bottom, in which case however an inner circular
portion and an outer portion in the form of a circular ring does not have
any coil windings in order to ensure that neither the container corner nor
the centre of the container bottom are directly heated. In general a
suitable drive for the container is disposed in the centre of the container
bottom. As the drive or lubricating greases used therein are frequently
heat-sensitive, that region is excluded in terms of the dimensioning of the
heating device for the container bottom. In another particularly preferred
embodiment the drive region is additionally screened in relation to the
electromagnetic alternating field of the coil.
In an alternative embodiment, alternatively or in combination a
container closure can be inductively heated. In a similar manner the
heating device for heating the container wall can be so arranged that a
lower portion of the container wall, in the form of a cylindrical surface, is
not substantially heated by the heating device, where the lower portion in
the form of a cylindrical surface is preferably of a height which is greater
than 5 % and particularly preferably greater than 10 % of the container
wall height. That arrangement avoids excessive heating of the container
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corner due to field superimpositioning with the simultaneous arrangement
of a heating device at the container bottom.
In addition the heating device for heating the container wall can be
so arranged that an upper portion of the container wall, in the form of a
cylindrical surface, is not substantially heated by the heating device,
wherein the upper portion in the form of the cylindrical surface is preferably
of a height which is greater than 10 % and particularly preferably greater
than 20 % of the container wall height. More specifically, the mixing
container is frequently not completely filled with material to be mixed so
that the described measure prevents the part of the container wall which is
not in contact with the material to be mixed being directly heated.
In a further embodiment the upper portion of the container wall, in
the form of a cylindrical surface, can comprise an electrically non-
conductive material and/or a material of very low thermal conductivity.
A further particularly preferred embodiment provides that provided
between the outside of the container on the one hand and the excitable coil
of the at least one heating device on the other hand is an electrically non-
conducting, preferably non-metallic insulation element which is preferably
secured to the outside of the container. In that case the insulation element
is best glued to the outside of the container. That insulation element
serves to reduce the delivery of heat from the container to the
environment. That saves on energy and reduces the expenditure for
affording protection from being touched.
The windings of the coil for the heating device associated with the
container bottom can be arranged for example in the proximity of the
outside of the container bottom and extend in a spiral around the axis of
rotation of the container. That arrangement ensures uniform heating of the
container bottom. Alternatively the windings can also be arranged only in a
segment of a circle. As the container rotates relative to the windings the
entire container bottom is then also heated. Admittedly that arrangement
results in less uniform heating of the container bottom, but it can be easily
retrofitted to existing mixing devices. In that respect the centre point
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angle of the segment of the circle is preferably less than 180 and
particularly preferably less than 90 and best less than 450
.
Equally the windings of the coil of the heating device associated with
the container wall can be arranged in the proximity of the container wall
and extend substantially concentrically around the container wall so that
substantially the entire container wall, with the exception of the free upper
and lower wall portions, can be heated.
In this case also alternatively the windings can also be arranged only
in the region of a container wall portion. The rotation of the container
ensures that the entire wall can be heated. This arrangement can be more
easily retrofitted in existing mixing devices.
Besides the possibility of providing just the wall or just the bottom of
the mixing container with a heating element, it would be possible for
example to provide a substantially L-shaped heating element, in the two
limbs of the L-shape of which is arranged a respective coil. That L-shaped
heating element is arranged with one limb parallel to the container wall and
the other limb parallel to the container bottom.
For many situations of use it may be advantageous if there is at least
one temperature sensor for measuring the container bottom temperature,
the temperature sensor preferably being an infrared sensor and detecting
the temperature on the outside of the container bottom.
In addition there can be provided at least one temperature sensor for
measuring the container wall, in which case also the temperature sensor is
preferably an infrared sensor and detectors the temperature on the outside
of the container wall. When using infrared sensors the insulation which is
possibly provided can have openings so that the temperature of the
container bottom and/or the container wall can be detected directly by
means of the infrared sensors.
In principle it is possible for the temperature of the container bottom
to be very precisely set, with the assistance of the corresponding
temperature sensor, by means of the heating device for the container
bottom. The temperature of the container wall can also be correspondingly
regulated in the same manner. To minimise deformation of the container
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because of thermal expansion regulation is preferably effected in such a
way that the temperature difference between the container bottom on the
one hand and the container wall on the other hand is as small as possible.
In a preferred embodiment the excitable coil is at least partially
shielded outwardly. That shielding is preferably effected by means of a
ferrite shielding.
Accordingly, in one aspect the present invention resides in a mixing
device having a rotating container for accommodating material to be
mixed which is provided in an inclined manner with respect to a horizontal
plane in a housing with a housing cover, at least one mixing tool which is
arranged in the interior of the container and which can be driven by a
motor, and heating devices for heating the material to be mixed,
characterized in that: at least one stripping device is arranged along a
wall of the container and/or a bottom of the container on the upper side of
the inclination of the container, and is designed to relatively move with
respect to the rotating container wall; the container at least partially
comprises an electrically conductive material, and at least two separately
regulatable heating devices respectively have at least one coil which can
be excited by an electric alternating field and which is so arranged that
eddy currents are produced in the electrically conductive material of the
container by the magnetic field change which occurs when the current
flow changes; at least one of the heating devices is designed to heat the
container wall and at least one of the heating devices is designed to heat
the container bottom; and the heating devices are provided as L-shaped
heating elements, one side of L of the L-shaped heating elements being in
a shape covering a portion less than 1800 of a form of a cylindrical surface
of the container wall.
Further advantages, features and possible uses of the present
invention will be apparent from the description hereinafter of preferred
embodiments and the accompanying Figures in which:
Figure 1 shows a first embodiment of the invention,
Figure 2 shows a second embodiment of the invention,
Figure 3 shows a third embodiment of the invention, and
Figure 4 shows a fourth embodiment of the invention.
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Figure 1 shows a first embodiment of the mixer 1 according to the
invention. The mixer 1 has a container 2 and a mixing tool 3 which is
arranged in the container 2 and can be driven by means of a motor 4.
The mixing tool 3 is arranged asymmetrically in the container 2.
The container 2 is arranged within a housing 7 having a housing
cover 6. It is also possible to see a stripping device 5 fixed to the housing
cover 6.
The container 2 is for example of metallic material and is
surrounded by an insulating layer 8. Arranged in the housing 7 and
substantially parallel to the container bottom and the container wall are
two coils 9 and 10 to which an ac voltage can be applied by way of the
feed lines 11 and 12 by the power supply 13. In addition suitable cooling
media for the dissipation of heat of the lost power in the coil can be
transported by way of the same or also separate feed lines. The ac
voltage which is transmitted by way of the feed line provides that eddy
currents are generated in the container 2, which in turn lead to a rise in
the temperature of the container. Both coils 9 and 10 are shielded at
least outwardly, preferably also towards the edges, by means of a ferrite
shielding 14. That ferrite shielding 14 serves to avoid possible eddy
current generation and thus an increase in temperature outside the
container.
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The heating device 9 associated with the container wall does not
extend over the entire container height. An upper region of the container
wall which is generally not acted upon with material to be mixed is not
heated. In the same manner the heating device 9 also does not extend as
far as the corner of the container to avoid excessive heating of the
container in the corner.
The heating device 10 for the container bottom is also not taken as
far as the corner. In addition a circular central region of the container, by
way of which the container is generally driven, is not heated. A first
temperature sensor 15 is provided on the stripping device 5 to determine
the temperature of the material to be mixed. In
addition infrared
temperature sensors are arranged in the region of the container wall and in
the region of the container bottom. To
permit exact temperature
measurement the insulation 8 has corresponding openings.
Figure 2 shows a second embodiment of the mixing device according
to the invention.
This embodiment differs from the embodiment shown in Figure 1
only in that two heating devices 9', 9" are provided to heat the container
wall. The two heating devices 9', 9" can both be actuated separately from
each other by way of the connections 12', 12" by the power supply 13.
Accordingly, two temperature sensors 17' and 17" are also provided
to detect the container wall temperature at two different positions. With
this arrangement the heating power levels can be individually adapted over
the height of the wall to the thermal power transmitted in the interior of the
container.
Figure 3 shows a third embodiment of the mixer according to the
invention. It
will be seen here that the mixer can be tilted about a
horizontal axis. To empty the mixer therefore firstly the housing cover 6
together with the mixing tool 3 and the stripping device 5 is tilted out of
the container. The container together with the material to be mixed is then
tilted towards the side so that the material to be mixed is transferred into a
truck 18 which is provided for those purposes. The heating device which is
of a L-shape configuration only comprises in the illustrated case one coil so
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that the wall and the bottom of the mixing container cannot be individually
regulated. A fixed, non-regulatable power distribution between the wall
and the bottom can be implemented for example by a different number of
windings for the coil in the wall and the bottom. The corner regions
5 between the wall and the bottom can also be substantially excluded from
the direct heating effect insofar as the corner region is correspondingly
cleared of the windings.
That represents a particularly inexpensive
embodiment of the invention.
Figure 4 shows a fourth embodiment of the invention. Unlike the
10 embodiment shown in Figures 1 and 2 here the coils extend around the
entire container and over the entire bottom surface respectively. This
embodiment produces uniform heating of the container 2. For
most
purposes of use however the arrangement of the embodiments 1 and 2 is
sufficient, especially as that can be retrofitted to existing mixing devices.
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List of references
1 mixer
2 container
3 mixing tool
4 motor
5 stripping device
6 housing cover
7 housing
8 insulating layer
9, 9', 9" coil/heating device
10 coil/heating device
11 feed line
12, 12', 12" feed line
13 power supply
14 ferrite shielding
15 temperature sensor
16 infrared sensor
17, 17', 17" infrared sensor
18 truck
